An MMSS (Multi-Mode Single-Standby) UE (User Equipment) with a single SIM (Subscriber Identity Module) has multiple RAT (Radio Access Technology) modules, such as modems (modulator-demodulator) for different RATs (which may be referred to as RAT modems), in which only one RAT module is supported in the standby or connected mode at the same time, where the RAT may be GSM (Global System of Mobile communication), WCDMA (Wideband Code Division Multiple Access), TD-SCDMA (Time Division-Synchronous Code Division Multiple Access), CDMA2000 (Code Division Multiple Access 2000), LTE (Long Term Evolution), WiMAX (Worldwide Interoperability for Microwave Access) or WiFi (Wireless Fidelity) technology.
The radio environment in the wireless communications system is more complicated than the wired environment in which to perform the TCP (Transmission Control Protocol) data transmission. The signal strength of the initial RAT may become worse when the UE moves. The data transmission, a data download, may need to switch to another RAT if the signal strength of the initial RAT drops to an unacceptable level. Although a telecommunication operator typically supports more than one RAT, the network coverage of certain RATs may not be sufficient. The insufficient coverage often leads to many blind areas, and the data transmission is interrupted when the UE attempts to conduct a RAT switch in a blind area. The conventional TCP recovery mechanism is developed on the basis of the wired environment. The UE may suffer greater packet loss when adopting the conventional TCP recovery mechanism because the UE takes more time to recover a radio link in a wireless communications system than it does in a wired environment.